How to properly test changing an API contract (adding a new mandatory field)?

When testing an API contract change with the addition of a new mandatory field, it is important to check several aspects:

1. Backward Compatibility Check: Ensure that clients not passing the new field receive correct errors or warnings.
2. Validation of the new field: Verify that the API correctly handles the absence, incorrect, and correct values of the new mandatory field.
3. Integration Testing: Check that all services interacting with the API handle the change correctly.
4. Documentation: Make sure the documentation is updated and reflects the new mandatory field.

Example of testing the addition of a mandatory field in JSON:

// Request without the new mandatory field
{
  "existingField": "value"
}
// Expected response: error indicating the missing field

// Request with the new mandatory field
{
  "existingField": "value",
  "newMandatoryField": "validValue"
}
// Expected response: successful processing

Automating such tests will help quickly identify regressions as the API evolves.